PROJECT SUMMARY The goal of this work is to improve the medical care of transgender women (TGW) through understanding the interaction of drugs for pre-exposure prophylaxis (PrEP) for prevention of HIV acquisition and gender affirming hormonal therapy (GAHT). Professional Societies recommend estrogen-based GAHTs for TGW to promote feminization and demasculinization in order to facilitate gender transition and diminish gender dysphoria. TGW are a vulnerable population, who experience significant challenges, including gender dysphoria, depression, and discrimination, all resulting in a compromised quality of life. There are barriers for transgender persons in terms of employment and healthcare, further preventing members of the community from receiving culturally competent care. In addition, TGW are 49-times at greater risk for HIV acquisition when compared to cisgender men (CGM) and women (CGW) of reproductive age. Currently, a daily, fixed dose formulation of tenofovir (TFV) disoproxil fumarate (TDF)/emtricitabine (FTC) (PrEP), has been used for the prevention of HIV acquisition in persons at high risk of HIV infection. A less-frequent, on demand, four-dose regimen has also proven highly effective in preventing HIV infection. While TGW have been enrolled in randomized controlled PrEP trials, TGW have not been enrolled in sufficient numbers to robustly evaluate PrEP uptake and efficacy. Importantly, the interaction of PrEP and GAHT drugs has not been well characterized. In vitro and ex vivo studies suggest an influence of estrogen and, potentially reduced testosterone, on PrEP pharmacology. Preliminary work by our group demonstrated greater than 30% reduction in plasma TFV and FTC AUC0-24 in TGW on estrogen when compared to CGM. Because reducing the weekly TDF/FTC dose from seven to two doses per week results in a sizeable drop in PrEP protection of anal sex from 90 to 75%, the change we observed may be sufficiently large to impact PrEP outcomes in TGW on GAHT. Thus, to better characterize the relationship between PrEP and GAHT, we propose a clinical drug-drug interaction between PrEP drugs and estrogen-based GAHT in a stepwise and estrogen dose-dependent manner to quantify the magnitude of drug interaction in blood, urine, and colorectal tissue. We will use these data, in conjunction with other clinical trials data, to build a population pharmacokinetic-pharmacodynamic model of PrEP efficacy. This will enable clinical trial simulations to contrast daily and on demand PrEP dosing in the presence and absence of GAHT to inform PrEP dose recommendations for TGW. Because we hypothesize that GAHT will decrease PrEP concentrations and result in increased susceptibility for HIV infection, TGW on GAHT will likely require higher or more frequent PrEP dosing than CGM who have sex with men. This work is a critical next step in ensuring appropriate PrEP dosing to group at substantially higher risk of HIV acquisition, and informs the need and design for future study of PrEP and GAHT from an efficacy, behavioral, and acceptability perspective.